Generally, a planetary reduction gear device used in a starter is constructed of a sun gear, an internal gear and planetary gears. The sun gear is provided on a rotation shaft of a motor. The internal gear is in a form of ring and is arranged concentric with the sun gear. The planetary gears are rotatably supported by gear shafts and engage with the sun gear and the internal gear.
When the sun gear rotates with the motor, the planetary gears rotate and revolute. The revolution movement of the planetary gears are transmitted to a planet carrier through the gear shafts, thereby to rotate the planet carrier. Accordingly, the rotation force of the motor is outputted from the planet carrier after the rotation speed is reduced by the revolution movement of the planetary gears. This kind of planetary reduction gear device is for example disclosed in Japanese Unexamined Patent Publication No. JP-A-58-77952.
In recent years, an engine needs to produce high power output and improve fuel efficiency. With this, the starter is required to vary an internal speed reduction ratio so as to meet a wide range of engine loads. In the above planetary reduction gear device, however, the gear shafts are arranged on a circle having a center that is coincident with a rotation axis of the planet carrier. Therefore, specifications of planetary gears, which can be mounted on the planet carrier, are limited. That is, it is difficult to change the speed reduction ratio by mounting planetary gears having different gear specification to the same planet carrier.
Also, the gear shafts are integrally formed into the planet carrier. A cross-sectional area of the gear shafts is generally smaller than that of the carrier. Therefore, when the planet carrier and the gear hafts are formed by cold forging, because the cross-sectional areas are different largely, a stress of a die is increased. As a result, the die is likely to be broken. Also, processing accuracy of the gear shafts, that is, accuracy of the diameter and length of the gear shafts, is likely to be reduced.